In article , Andrew Gabriel
writes

Having thought some more, I have an idea. I've been experimenting
with how low I can set the boiler temperature, the ideal being to
set it as low as possible so the boiler doesn't cycle on and off
via the roomstat, but runs continuously. This should give the most
efficient condensing operation. Over the last few months, I've never
had the flow temperature set above 50C, and for much of that it was
only 45C. This isn't hot enough to kill off all bugs, fungii, etc.
As others have commented, organic material does get sucked in -- some
of it I find desicated in the bottom of the casing, but some will
go on though the burner. This will leave organic debris at the bottom
of the heat exchanger chamber. Now with the temperature at only 45C,
it may be that some molds or other things can live there growing on
this residue. A lump of something growing submersed bacteria/mold/algae
on it might explain the jellyfish like thing I found bits of.

Well, it's the only theory I can think of at the moment.

Having had the burner out, I got to wondering what it looks like
operating (there's no way to see it as there's no observation
window). It was something like 3x8" in size, probably similar area
to a radiant gas fire of perhaps 3kW. However, this element is 25kW,
so it's probably quite spectacular if you could actually see it
operating.

I run mine pretty low too, but I've modified it to run full whack for hot water
demand, let me know if you want the circuit (after christmas :-). Mind you,
it also runs full whack for 10mins or so at startup, but maybe that's not
long enough to kill off the slime.

I didn't bother priming the condensate drain, no tundish, plumbed into a
closed drain and testing showed it would half fill a bucket in a day so I
reckoned the trap would fill in an hour or so of running. Also the CO
content at the outlet was tiny as I'd just set it up ;-).
--
fred